Real time communications systems, such as push-to-talk communications systems, require that members of a talk group know the status and location of other members of a talk group. In order for such a communication system to work effectively, the information regarding each member's status and availability must be delivered to other members in real time. For example, if a subscriber activates his or her push-to-talk handset and becomes available, it is necessary to immediately distribute this information to all of the other users in the talk group.
One protocol that may be used to communicate subscriber status and location information among members of a talk group is the presence protocol. The presence protocol is described in IETF RFCs 2778 and 2779. According to the presence protocol, subscriber status information is stored in a database, referred to as a presence database. Other subscribers may subscribe to a first subscriber in the database. When the status of the first subscriber changes, the presence database sends notification messages to the other subscribers to notify the subscribers of the change in status. The other subscribers use this change in status to determine whether and where the first subscriber is available to receive communications.
One method for distributing presence protocol messages to mobile telecommunications network subscribers is to use short message service (SMS). SMS allows subscribers to send short text messages to each other via their mobile handsets. Subscribers can also send SMS messages to application servers, such as a presence, Push-to-Talk (PTT) or other server. SMS relies on short message service centers (SMSCs) to ensure that SMS messages are delivered. An SMSC stores SMS messages, locates destination subscribers, and forwards the SMS messages to the destination subscribers when they become available. SMSCs may reattempt delivery if a subscriber is initially unavailable.
One problem with using SMS to deliver presence information regarding push-to-talk subscribers is that presence information regarding push-to-talk subscribers loses its utility after a short time period. For example, if the intended recipient of an SMS message containing presence information regarding another subscriber is unavailable when the SMS message is initially generated, the SMSC stores the message and attempts re-delivery at a later time. Using the store and forward mechanism currently provided by SMSCs wastes SMSC resources if the intended recipient of the presence information is unavailable and introduces latency in delivering presence information when the intended recipient is available. Accordingly, there exists a need for improved methods and systems for delivering presence information regarding push-to-talk subscribers.